The invention relates to a method of printing on cylindrical objects by resorting to the following steps:
Coating the surface of a cylindrical object with a migration-preventing plastic material that has an affinity for dye, such as printing ink;
Advancing cylindrical objects continuously in a row, one after the other, through an ink application zone in which printing inks are transferred from an endless flexible carrier onto the surfaces of the objects in that the endless, flexible carrier of printing ink, which sublimes in the heat, contacts under tensile stress a portion of the surface of each cylindrical object;
Rolling the cylindrical objects relative to the carrier during advancement through the ink application zone; and
Heating at least the auxiliary carrier above the sublimation temperature of the printing inks.
The invention also relates to an apparatus for implementation of the method.
A method of the above outlined character and an appurtenant apparatus are known from published German Pat. application Ser. No. 32 29 815. This publication proposes to advance the cylindrical objects, which are contacted by the ink-bearing carrier, through a heating zone. In the heating zone, the objects and the carrier are heated up so that the printing inks can migrate from the carrier into and diffuse in the plastic layers at the surfaces of the objects. With the known method and the known apparatus, objects can be printed in a highly satisfactory manner. The method is being used in particular for printing on beverage cans with a detailed motif, that is, on beverage cans whose external surfaces bear the name of the manufacturer and identify the contents of the can, together with relatively detailed motifs, similar to printed labels on beverage bottles.
It has been ascertained that the quality of printing in accordance with the proposal in the published German patent application is indeed quite high but that the appearance of the cans will suffer after even slight damage to the plastic layer, e.g., as a result of small scratches on the surface. When handling the cans, for example during filling in the bottling plant of a beverage production facility, where several thousand cans must be handled every hour, such damage cannot always be reliably prevented. As a rule, the damage does not amount to removal of plastic layer down to the bare metal of the can. Rather, the damage is in the form of light scratches or similar flaws which affect primarily the image of the print because, as a rule, a portion of the dye is removed. In order to reduce the adverse effects caused by such scratching to a minimum, attempts were made to introduce the ink deeper into the plastic layer. This can be achieved, for instance, by increasing the temperature in the heating zone. However, an increased amount of energy is required to increase the temperature, and this results in increased costs.